Noise reduction system for actively compensating background noise, method of operating the system and use of the system

ABSTRACT

A noise reduction system for actively compensating background noise in a passenger transport area of a vehicle. The system includes an averaging unit configured to calculate an average error signal, which is indicative of a difference between a background noise and an anti-noise at more than one position in a noise reduction area and a dynamic adjustment unit, configured to update parameters of the anti-noise unit based on the average error signal and so as to minimize the average error signal.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is based upon and claims the benefit of priorityfrom DE 10 2022 118 016.6 filed on Jul. 19, 2022, the entire contents ofwhich is incorporated herein by reference.

BACKGROUND Field

The present disclosure relates to a noise reduction system and moreparticularly to a noise reduction system for actively compensatingbackground noise generated by a noise source in a noise reduction areain a passenger transport area of a vehicle.

Further, the present disclosure relates to a method of operating a noisereduction system for actively compensating background noise generated bya noise source in a noise reduction area in a passenger transport areaof a vehicle.

Prior Art

Noise reduction systems are known in various configurations. Noisereduction systems are also referred to as noise suppression systems,background noise suppression systems, background noise reduction systemsand noise-canceling systems. A distinction is made between active andpassive systems. In case of a passive system, sound-absorption materialsare applied in order to reduce the undesired background noise in forexample a passenger area of a vehicle. In active noise reductionsystems, which are also referred to as active noise-canceling systems oractive noise control systems (often abbreviated as “ANC”), active noisecompensation by means of anti-noise (also referred to as “counternoise”) is applied. The anti-noise is superimposed on the undesiredbackground noise in that the background noise is reduced or almostcompletely eliminated in a quiet zone by means of destructiveinterference.

In the context of this disclosure, only active noise reduction systemsare explained, even if these are not explicitly referred to as activenoise reduction systems but rather merely as noise reduction systems.

In noise reduction systems, efficient suppression of the backgroundnoise can only be achieved within a small spatial region. This spatialregion is typically referred to as a quiet zone and lies inside a noisereduction area of the system. In the quiet zone, the anti-noise issuperimposed on the background noise in more or less exact phaseopposition. Therefore, efficient suppression of the background noiseoccurs. This spatial limitation leads to the effect that active noisereduction systems are rather sensitive to movements of the head of auser. When the entrance of the auditory channel at the ear of the useris no longer located in the quiet zone, efficient background noisereduction cannot be guaranteed and the noise reduction system loseseffectiveness.

This is why a relocation or readjustment of the noise reduction area isperformed in many cases. Generally, noise reduction systems are drivenby minimizing an error signal, which indicates the residual noise notcanceled by the noise reduction system. To provide efficientnoise-canceling, the residual noise near or at the auditory channel ofthe ear of the user should be minimized. To estimate said noise at aposition in which no physical microphone can be placed or is not desiredto be placed, the concept of “virtual microphones” has been established.This concept is basically described for example in U.S. Pat. No.5,381,485.

When referring back to the movement of the user's head, the adaption ofthe noise reduction system to said movement is performed by relocating aposition of the virtual microphone, which is configured to pick up thesum of the background noise and the anti-noise.

In many cases, a microphone array is applied for picking up a signalused for subsequent estimation of the signal at the position of thevirtual microphone. There are various approaches applying differentfilters that are used to estimate a residual signal representing the sumof the background noise and the anti-noise at a position of the virtualmicrophone.

Furthermore, an active noise reduction system comprises a microphone fordetecting the background noise of a noise source, the noise of whichshould be eliminated in the noise reduction area. This microphone isoften referred to as a reference microphone. An anti-noise filterdriving a sound generator that emits the anti-noise uses the signal ofthe reference microphone. The output of the anti-noise filter is notonly used for driving the sound generator but is also input to a furtherfilter. This is configured to estimate a muting signal representing theanti-noise at the position of the before mentioned virtual microphone.By subtracting the estimated muting signal from the estimated signal,which is the background noise and the anti-noise, an error signal can bederived. This error signal represents a cost function of the noisereduction system. By minimizing the value of the error function, thenoise-canceling system is dynamically adapted to the noise generated bythe noise source and by that, efficient noise reduction at the positionof the virtual microphone can be achieved.

The position of the virtual microphone does however not match in allsituations with the location of the auditory channel of the user's ear.In an attempt to provide a flexible and dynamic noise reduction in anoise reduction area, a plurality of virtual microphones can beestablished. A virtual microphone can be selected for active noisereduction, wherein a selection of the virtual microphone being locatednext to the detected location of the user's ear will provide the mostefficient noise cancelation. Systems using a plurality of virtualmicrophone positions are for example known from EP 3 435 372 A1 or fromWO 2020/047286 A1. The analysis of a plurality of virtual microphonepositions however places a significant computational load on the controlunit of the noise reduction system.

SUMMARY

In view of the above, it is an object to provide a noise reductionsystem for actively compensating background noise, a method of operatingsuch a system and use of said system, wherein efficient noise reductionin a noise reduction area should be provided while lowering thecomputational load on a control unit of the system.

Such object can be solved by a noise reduction system for activelycompensating background noise generated by a noise source in a noisereduction area in a passenger transport area of a vehicle, the systemcomprising a controller, a reference sensor for detecting the backgroundnoise of the noise source, a sound generator for generating anti-noisefor superimposing the anti-noise with the background noise in the noisereduction area for active reduction of the background noise, and amonitor-microphone array having a plurality of monitor microphones, themonitor-microphone array being disposed adjacent to the noise reductionarea and being configured to pick up background noise emitted by thenoise source and anti-noise emitted by the sound generator,

-   -   wherein a virtual sensing algorithm is implemented in the        controller, which is configured to estimate an error signal at a        position of a virtual microphone, wherein the virtual microphone        is located in the noise reduction area and the error signal is        indicative of a difference between the background noise and the        anti-noise at the position of the virtual microphone, the        controller further comprising an anti-noise unit for generating        an anti-noise signal for driving the sound generator in that it        generates the anti-noise,    -   and wherein the controller further comprises an averaging unit        configured to calculate an average error signal, which is        indicative of a difference between the background noise and the        anti-noise at more than one position in the noise reduction area        and a dynamic adjustment unit, which is configured to update        parameters of the anti-noise unit based on the average error        signal and so as to minimize the average error signal.

In this noise reduction system, the quiet zone can be enlarged. A largerquiet zone provides more flexibility for a potential movement of a headof a user and thereby significantly increases the effect and usabilityof a noise reduction system. Furthermore, the noise reduction system canreduce the computational load on the control unit. Traditional noisereduction systems apply multiple error microphone paths, wherein aseparate noise reduction algorithm is run for every virtual microphonepath. These systems are MIMO systems (Multiple Input Multiple Output)and demand for high computational power to calculate the multiple errorpaths in parallel. While these systems can be capable of increasing thequiet zone to some extent, this performance is achieved at the cost of avery high computational load for the analysis of the multiplicity oferror signals. A significant computational burden is placed on thesystem hardware. This renders the system cost-intensive.

Within the context of this disclosure, the difference between thebackground noise and the anti-noise, which is the error signal, isindicative of a residual noise, which is not cancelled by the noisereduction system. The position, for which said difference is calculated,can be a position of a virtual microphone. The calculation for more thanone position implies that the calculation is performed in that saiddifference is calculated for more than one position of a virtualmicrophone, i.e. for example for a plurality of virtual microphone orfor a spatially extended virtual microphone.

According to an embodiment, the noise reduction system can comprise thefeature, according to which a plurality of positions can be located inthe noise reduction area and the control unit can be configured toestimate at least a first error signal for a virtual (error) microphonelocated at a first position and a second error signal for a virtual(error) microphone located at a second position and the averaging unitcan be configured to calculate the average error signal from at leastthe first and the second error signal.

The above outlined technical drawbacks can be overcome by the noisereduction system disclosed herein. An aspect can be the averaging of theerror signal, wherein an average of two or more virtual (error)microphones=can be taken into account. By this measure, thecomputational load can be reduced to a level, which is similar tosystems using only a single virtual microphone. At the same time, thequiet zone can be significantly increased. This can represent asignificant advantage for practical implementation of the system.

The virtual microphones in the noise reduction area can be arranged in agrid or the position of the virtual microphones can be freely selectedand defined. It is also possible to dynamically rearrange the virtualmicrophones, which means that their positions can be changed oroptimized during operation of the system. There can be multiplicity ofpredetermined positions, at which the virtual microphones can be placed.

According to an embodiment, the noise reduction system can be furtherenhanced in that the averaging unit can be further configured tocalculate an average error signal, which can be an arithmetic average ofthe at least first and second error signal. The computation of theaverage error signal by calculating the arithmetic average can be asimple and efficient way for determination of the average error signal.

According to an alternative embodiment, the averaging unit can befurther configured to calculate an average error signal, which can be aweighted average of the at least first and second error signal.Calculating a weighted average of the error signals can allow to focuson one or more positions in the noise reduction area. This can beperformed by overemphasizing the respective virtual microphone. Theemphasis can be put on a single virtual microphone or one more than onemicrophone, depending on whether particular emphasis should be put onone or on more than a single position. This can result in a shift of thequiet zone to a certain area or to certain areas, while the overall areaof the quit zone can be increased. The emphasis can be put on saidvirtual microphone by overweighting the signal of this microphone in thecalculation of the weighted average. For example, the signal can bemultiplied by a factor greater than one, which defines the overweight,while the remaining signals are considered without a factor, whencalculating the weighted average.

The noise reduction system, according to a further embodiment, cancomprise a position detection unit, for example a camera arrangement,which can be configured to detect a position and/or orientation of ahead of a passenger and to estimate a position of an ear of a passengerin the passenger transport area, wherein the control unit can be furtherconfigured to select a main position of the plurality of positions forthe virtual microphones, which can be adjacent to, such as closest to,the estimated position of the ear of the passenger, wherein theaveraging unit can be configured to overweight the error signal at themain position when calculating the average error signal.

The position detection unit can be configured as a head tracking system.Using this system, the quiet zone can follow the movement of thepassenger's head. This can be performed by shifting the position of thevirtual microphone, which can be overemphasized in the calculation ofthe weighted average. If the virtual microphones are arranged in a fixedpattern or grid, the virtual microphone being located nearest to theauditory channel of a user's ear, can be selected as the microphone onwhich particular emphasis is put. When calculating the weighted average,this particular virtual microphone can be overemphasized.

According to another embodiment, the controller can comprise: a firstfilter unit configured to receive the anti-noise signal and to estimatea shifted anti-noise signal, which can be indicative of the anti-noiseat a physical position of one of the monitor-microphones of themonitor-microphone array, a first arithmetic unit configured to receivethe shifted anti-noise signal and a monitor signal of the monitormicrophone being located at said physical position, wherein the firstarithmetic unit can be configured to calculate a residual signal, whichcan be a difference between the monitor signal and the shiftedanti-noise signal at the physical position of the monitor microphone, asecond filter unit, which can be configured to receive the residualsignal and to estimate a shifted residual signal, which can be theresidual signal shifted to the position of the virtual microphone, athird filter unit configured to receive the anti-noise signal and toestimate a shifted anti-noise signal, which can be indicative of theanti-noise at the position of the virtual microphone, a secondarithmetic unit configured to receive the shifted residual signal andthe shifted anti-noise signal and to estimate the error signal for theposition of the virtual microphone by addition of the shifted residualsignal and the shifted anti-noise signal.

The virtual sensing algorithm in the control unit can be implementedaccording to the remote microphone technique. This has been provenadvantageous in practical experiments because it provides the bestperformance under the desired circumstances.

According to further embodiments, the virtual sensing algorithm can beimplemented by other means. For example, the controller can comprise avirtual sensing algorithm which can be a virtual microphone arrangement,a forward difference prediction technique, an adaptive LMS virtualmicrophone technique, a Kalman filtering virtual sensing algorithm or astochastically optimal tonal diffuse field virtual sensing technique.One of these algorithms can be implemented in the controller accordingto further embodiments. Without prejudice, further reference will bemade to the preferred embodiment, which is the implementation of theremote microphone technique, in the following.

Furthermore, the noise reduction system can be further enhanced in thatthe first filter unit, the first arithmetic unit, the second filterunit, the third filter unit and the second arithmetic unit can beconfigured to calculate and estimate the respective signals for at leastthe first and the second position, such as for all positions in thenoise reduction area.

By taking into account a plurality of or all virtual microphones, whichcan be located in the noise reduction area, the quiet zone can bemaximized. Furthermore, a maximum flexibility with respect to headtracking and weighting during calculation of the average error signalcan be provided.

According to another embodiment, the calculation is not performed for aplurality of points at which the virtual microphone can be placed, butright from the beginning, the calculation can be based on apredetermined section of the noise reduction area, which can be asubarea thereof.

According to another embodiment, the averaging unit can be configured tocalculate an average error signal, which can be indicative of adifference between the background noise and the anti-noise in a sectionof the noise reduction area comprising more than one position.

A practical implementation of this system according to the remotemicrophone technique leads to the following embodiment according towhich the averaging unit can be configured to receive a plurality ofmonitor signals of monitor microphones being located at differentphysical positions and to estimate an area monitor signal, which cam beindicative of an error signal captured by the monitor microphones for apredetermined area of the monitor microphones, wherein the controllercan comprise: a first filter unit configured to receive the anti-noisesignal and to estimate a shifted area anti-noise signal, which can beindicative of the anti-noise in the predetermined area, a firstarithmetic unit configured to receive the shifted area anti-noise signaland the area monitor signal, wherein the first arithmetic unit can beconfigured to calculate an area residual signal, which can be adifference between the area monitor signal and the shifted areaanti-noise signal, a second filter unit, which can be configured toreceive the area residual signal and to estimate a shifted area residualsignal, which can be the area residual signal shifted to a predeterminedvirtual area comprising more than one position of a virtual microphone,a third filter unit configured to receive the anti-noise signal and toestimate a shifted area anti-noise signal, which can be indicative ofthe anti-noise in the predetermined virtual area, and the averaging unitfurther comprises a second arithmetic unit configured to receive theshifted area residual signal and the shifted area anti-noise signal andto estimate the error signal for the predetermined virtual area as theaverage error signal by addition of the shifted area residual signal andthe shifted area anti-noise signal.

The area based calculation can reduce the computational load while, atthe same time, the quiet zone can be enlarged. On the other hand, theaveraging over the plurality of error values for different positions inthe quiet zone can allow a dynamic and flexible adaption of the system,wherein emphasis can be put on various points in the noise reductionarea. While the area based calculation can be more a static solution,the average based solution using a plurality of error values can bedynamically adapted on the individual use situation for example incombination with head tracking.

The noise reduction system according to another embodiment can befurther enhanced in that the monitor-microphone array can furthercomprise a direct monitor microphone and the averaging unit can beconfigured to calculate the average error signal, by further taking intoaccount a residual signal of the direct monitor microphone.

The signal of the direct monitor microphone can act as a physicalsupport vector for the calculation, which can be based on the estimatedsignals of the virtual microphones. Although this measure iscounterintuitive, it could be found that this measure enhances therobustness of the algorithm. It could further be found that the signalof the direct monitor microphone compensates for estimation errors inthe signals of the virtual microphones. The algorithm using weightedcontrol can be more robust when compared to traditional solutions.

The concept of the direct monitor microphone can be combined with eitherone of the two aforementioned embodiments, namely the averaging over aplurality of error signals and the area based approach.

When referring to the averaging over the plurality of error signals,this leads to the following embodiment, according to which the monitormicrophone array can further comprise a direct monitor microphone andthe averaging unit can be configured to calculate the average errorsignal, by further taking into account a direct residual signal of thedirect monitor microphone.

According to the area based approach, the noise reduction system can befurther enhanced in that the averaging unit can be configured to bypassa direct monitor signal of the direct monitor microphone, the firstarithmetic unit can be further configured to receive the shifted directanti-noise signal and a direct monitor signal of the direct monitormicrophone, wherein the first arithmetic unit can be configured tofurther calculate a direct residual signal, which can be a differencebetween the direct monitor signal and the shifted direct anti-noisesignal at the position of the direct monitor microphone, the firstarithmetic unit can be further configured to receive the shifted directanti-noise signal and a direct monitor signal of the direct monitormicrophone, wherein the first arithmetic unit can be configured tofurther calculate a direct residual signal, which can be a differencebetween the direct monitor signal and the shifted direct anti-noisesignal at the position of the direct monitor microphone, the secondfilter unit and the second arithmetic unit can be configured to bypassthe direct residual signal and the averaging unit can be furtherconfigured to calculate the average error signal, which can be anaverage of the error signal for the predetermined virtual area and thedirect residual signal.

Furthermore, the noise reduction system can be enhanced in that thecontrol unit can further comprise at least one band pass unit, which canbe configured to apply a band pass filter on the average error signaland/or on a noise signal picked up by the reference sensor for detectingthe background noise of the noise source.

The band pass filter can be a band pass for the frequency range between50 Hz and 600 Hz. Furthermore, it can be a low-pass filter, wherein acutoff frequency of the low-pass filter is between 400 Hz and 1000 Hz,such as between 500 Hz and 800 Hz, or at least approximately 600 Hz. Theupper cutoff frequency can be chosen in that a prefix of the anti-noisesignal does not change within the noise reduction area. Thisprerequisite can be advantageous for the stability of thenoise-canceling algorithm. When calculating a spatial distance from afrequency in one of the mentioned ranges, applying the well-knownformula by further taking into account the speed of sound, this resultsin a spatial distance of about 0.2 m. This limit should be a maximumdistance for the points at which the virtual microphones are arranged.The same applies for a distance between the point at which the virtualmicrophone can be arranged, i.e. one of the aforementioned points, andthe physical position of the direct microphone.

According to another embodiment, the band pass filter can be adjusteddynamically. This can be performed based on information of the size ofthe desired quiet zone. When the size of the desired quiet zone or amaximum distance between points for the virtual microphone or pointsbetween the most distant virtual microphone position and the directmicrophone are known, a maximum frequency used in the noise-cancelingalgorithm can be determined. By limiting the algorithm to this maximumfrequency, stability of the noise-canceling effect in the noise area canbe significantly enhanced. In other words, by using the band passfilter, the stability of the algorithm can be further enhanced. In otherwords, the band limited control enhances the proper working of the noisereduction system. The band limited control comes into effect becauseefficient destructive interference, which can be essential for efficientnoise-canceling, does typically only take place in a spatial area, whichis about one tenth of the wavelength under consideration, i.e. thewavelength to be canceled.

The noise reduction system according to an embodiment can providecontrol using a weighted average and thereby enhances the noisecancelation effect. This effect is not limited to a fixed head positionbut can be flexibly adapted to the movement of the user's head byapplication of head tracking technology. By further taking into accountthe direct signal of a physical microphone, the algorithm's stabilitycan be significantly enhanced.

Such object can be further solved by a method of operating a noisereduction system for actively compensating background noise generated bya noise source in a noise reduction area in a passenger transport areaof a vehicle, the system comprising a controller, a reference sensor fordetecting the background noise of the noise source, a sound generatorfor generating anti-noise for superimposing the anti-noise with thebackground noise in the noise reduction area for active reduction of thebackground noise, and a monitor-microphone array having a plurality ofmonitor microphones, the monitor-microphone array being disposedadjacent to the noise reduction area and being configured to pick upbackground noise emitted by the noise source and anti-noise emitted bythe sound generator, wherein a virtual sensing algorithm is implementedin the controller, which thereby estimates an error signal at a positionof a virtual microphone, wherein the virtual microphone is located inthe noise reduction area and the error signal is indicative of adifference between the background noise and the anti-noise at theposition of the virtual microphone, the controller further comprises ananti-noise unit for generating an anti-noise signal for driving thesound generator in that it generates the anti-noise, wherein this methodis further enhanced in that the controller further comprises anaveraging unit, which can calculate an average error signal, which canbe indicative of a difference between the background noise and theanti-noise at more than one position in the noise reduction area and adynamic adjustment unit, which updates parameters of the anti-noise unitbased on the average error signal, so as to minimize the average errorsignal.

Furthermore, the method can comprise a plurality of positions arelocated in the noise reduction area and the controller estimates atleast a first error signal for a virtual microphone located at a firstposition and an second error signal for a virtual microphone located ata second position and the averaging unit calculates the average errorsignal from at least the first and the second error signal.

According to an embodiment, the method comprises the feature accordingto which the averaging unit further calculates the average error signal,which is an arithmetic average of the at least first and second errorsignal.

According to yet another embodiment, the method can be further enhancedin that the averaging unit can calculate the average error signal, whichcan be a weighted average of the at least first and second error signal.

According to another embodiment, the method can be further enhanced inthat the noise reduction system further can comprises a positiondetection unit which detects a position and/or orientation of a head ofa passenger and estimates a position of an ear of a passenger in thepassenger transport area, wherein the controller can further select amain position of the plurality of positions, which can be adjacent to,such as closest to, the estimated position of the ear of the passenger,wherein the averaging unit gives an overweight to the error signal atthe main position when calculating the average error signal.

Furthermore, the method can be enhanced by the controller furthercomprising: a first filter unit, which receives the anti-noise signaland estimates a shifted anti-noise signal, which can be indicative ofthe anti-noise at a physical position of one of the monitor —microphonesof the microphone array, a first arithmetic unit, which receives theshifted anti-noise signal and a monitor signal of the monitor microphonebeing located at said physical position, wherein the first arithmeticunit calculates a residual signal, which can be a difference between themonitor signal and the shifted anti-noise signal at the physicalposition of the monitor microphone, a second filter unit, receives theresidual signal and estimates a shifted residual signal, which can bethe residual signal shifted to the position of the virtual microphone, athird filter unit, which receives the anti-noise signal and estimates ashifted anti-noise signal, which can be indicative of the anti-noise atthe position of the virtual microphone, a second arithmetic unit, whichreceives the shifted residual signal and the shifted anti-noise signaland estimates the error signal for the position of the virtualmicrophone by adding the shifted residual signal and the shiftedanti-noise signal.

According to yet another embodiment, the method can be further enhancedin that the first filter unit, the first arithmetic unit, the secondfilter unit, the third filter unit and the second arithmetic unit cancalculate and estimate the respective signals for at least the first andthe second position, such as for all positions in the noise reductionarea.

The method can be also further enhanced in that the averaging unit cancalculate an average error signal, which can be indicative of adifference between the background noise and the anti-noise in a sectionof the noise reduction area comprising more than one position.

Furthermore, the method can comprise the feature according to which theaveraging unit receives a plurality of monitor signals of monitormicrophones being located at different physical positions and estimatesan area monitor signal, which can be indicative of an error signalcaptured by the monitor microphones for a predetermined area of themonitor microphones, wherein the controller can comprise: a first filterunit, which receives the anti-noise signal and estimates a shifted areaanti-noise signal, which can be indicative of the anti-noise in thepredetermined area, a first arithmetic unit, which receives the shiftedarea anti-noise signal and the area monitor signal, wherein the firstarithmetic unit calculates an area residual signal, which can be adifference between the area monitor signal and the shifted areaanti-noise signal, a second filter unit, which receives the arearesidual signal and estimates a shifted area residual signal, which canbe the area residual signal shifted to a predetermined virtual areacomprising more than one position of a virtual microphone, a thirdfilter unit, which receives the anti-noise signal and estimates ashifted area anti-noise signal, which can be indicative of theanti-noise in the predetermined virtual area, and the averaging unitfurther comprises a second arithmetic unit, which receives the shiftedarea residual signal and the shifted area anti-noise signal andestimates the error signal for the predetermined virtual area as theaverage error signal by adding the shifted area residual signal and theshifted area anti-noise signal.

Furthermore, the method can be enhanced by the monitor-microphone array,which can further comprise a direct monitor microphone and in that theaveraging unit calculates the average error signal, by further takinginto account a direct residual signal of the direct monitor microphone.

According to another embodiment, the method can be further enhanced inthat the first filter unit further estimates a shifted direct anti-noisesignal, which can be indicative of the anti-noise at a physical positionof the direct monitor microphone, the first arithmetic unit furtherreceives the shifted direct anti-noise signal and a direct monitorsignal of the direct monitor microphone, wherein the first arithmeticunit further calculates a direct residual signal, which can be adifference between the direct monitor signal and the shifted directanti-noise signal at the position of the direct monitor microphone, thesecond filter unit and the second arithmetic unit bypass the directresidual signal and the averaging unit calculates the average errorsignal, which can be an average of the at least one error signal for aposition in the noise reduction area and the direct residual signal.

The method can also be further enhanced in that the averaging unit canbypass a direct monitor signal of the direct monitor microphone, thefirst arithmetic unit further receives the shifted direct anti-noisesignal and a direct monitor signal of the direct monitor microphone,wherein the first arithmetic unit further calculates a direct residualsignal, which can be a difference between the direct monitor signal andthe shifted direct anti-noise signal at the position of the directmonitor microphone, the first arithmetic unit further receives theshifted direct anti-noise signal and a direct monitor signal of thedirect monitor microphone, wherein the first arithmetic unit furthercalculates a direct residual signal, which can be a difference betweenthe direct monitor signal and the shifted direct anti-noise signal atthe position of the direct monitor microphone, the second filter unitand the second arithmetic unit bypass the direct residual signal and theaveraging unit further calculates the average error signal, which can bean average of the error signal for the predetermined virtual area andthe direct residual signal.

Furthermore, the method can be enhanced by the controller furthercomprising at least one band pass unit, which applies a band pass filteron the average error signal and/or on a noise signal picked up by thereference sensor for detecting the background noise of the noise source.

Same or similar advantages which have been mentioned with respect to thenoise reduction system apply to the method of operating the noisereduction system in a same or similar way and are therefore notrepeated.

Such object can also be solved by use of the noise reduction systemaccording to any of the previously mentioned embodiments. The used noisereduction system is for actively compensating background noise generatedby a noise source in a noise reduction area in a passenger transportarea of a vehicle. The vehicle can be a commercial vehicle, or aconstruction vehicle.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features of the embodiments will become apparent from thedescription of the embodiments together with the claims and the attacheddrawings. Embodiments can fulfill individual features or a combinationof several features.

The embodiments are described below, without restricting the generalidea of the invention, using exemplary embodiments with reference to thedrawings, express reference being made to the drawings with regard toall details that are not explained in greater detail in the text. In thedrawings:

FIG. 1 illustrates a simplified schematic drawing illustrating a vehiclecomprising a noise reduction system,

FIG. 2 illustrates a simplified schematic illustration of a noisereduction system and

FIGS. 3 to 6 illustrate drawings illustrating a functionality of a noisereduction system according to various embodiments.

In the drawings, the same or similar elements and/or parts are providedwith the same reference numbers in order to prevent the item fromneeding to be reintroduced.

DETAILED DESCRIPTION

FIG. 1 is a simplified schematic drawing of a vehicle 2, which can be apassenger car, a commercial vehicle, a construction vehicle or any otherroad driven vehicle. The vehicle 2 comprises a passenger transport area4, which is illustrated in dashed line. The vehicle 2 is equipped with anoise reduction system for actively compensating background noise, whichis generated by a noise source 6. The noise source 6 can be the engineof the vehicle 2 or any other device or source which generates undesiredbackground noise. For example, the noise source 6 can be a wheel, anauxiliary drive or a mechanic or hydraulic system of the vehicle 2. Thenoise, which is to be reduced in the passenger transport area 4 ismeasured by a sensor 8. The sensor 8 can be any device suitable fordetecting the background noise of the noise source 6. It can be amicrophone or an acceleration sensor. The sensor 8 is not limited to anelectro acoustical or electromechanical device like a microphone. It isalso possible to input a signal related to the background noise source 6to a model, which outputs a computed background noise signal. Forexample, a number of revolutions of an engine or any other suitableparameter thereof can be input to the model of the engine or can bedirectly input to the noise-canceling system. In other words, parametersof the noise source 6, which are electronically available, can bedirectly used for estimation of the background noise.

The noise reduction system of the vehicle 2 comprises a control unit 10(such as a processor/controller comprising hardware), which can be aseparate electronic device. The control unit 10, however, can also beimplemented as software in a main controller of the vehicle 2, which, inthis case, provides the control unit 10. The noise reduction systemfurther comprises a sound generator 12 for generating anti-noise. Thesound generator 12 can be a loudspeaker. The anti-noise and thebackground noise are superimposed in a noise reduction area 14 foractive reduction of the background noise. Furthermore, the noisereduction system comprises a monitor-microphone array 16, which isdisposed adjacent to the noise reduction area 14. The monitor-microphonearray 16 is configured to pick up background noise emitted by the noisesource 6 and anti-noise emitted by the sound generator 12.

FIG. 2 shows a simplified schematic illustration of the noise reductionsystem 20, which can be integrated in the vehicle 2 shown in FIG. 1 . Byway of an example, the main parts of the system are arranged in adriver's seat 22, such as in a headrest 24 of the seat 22.

There is the control unit 10, a plurality of monitor microphones 15forming the monitor-microphone array 16 and the sound generator 12.Furthermore, a sensor 8, for example a microphone, can be arranged inthe headrest 24 for detecting the background noise of the noise source 6(schematically represented by a loudspeaker). The senor 8 can also bearranged remote from the remaining parts of the system 20 as it is forexample illustrated in FIG. 1 . The noise reduction system 20 in FIG. 2is a compact system, which can be completely implemented in one singleunit, by way of an example in the headrest 24. In a more distributedsystem, it is also possible that the noise-canceling system 20 usesexisting sensors, which are already present in the vehicle 2 and areused by other systems of the vehicle 2, for example by an audio system.

The noise reduction system 20 can be used with or without the sensor 8.The presence of the sensor 8 depends on whether the noise reductionsystem 20 is a feed forward system (with the reference sensor 8) or afeedback system (without the reference sensor 8). If the system 20dispenses with the sensor 8, the background noise is directly detectedusing the monitor-microphone array 16. Furthermore, the noise reductionsystem 20 comprises a sound generator 12, which is for example aloudspeaker. The sound generator 12 is also located in the headrest 24by way of an example only.

The noise reduction system 20 further comprises a head tracking system26, which comprises for example a pair of stereo cameras 28. The headtracking system 26 is applied for detecting a position and/ororientation of the head 30 of a passenger, who is situated in thepassenger transport area 4. The head tracking system 26 is suitable fordetecting the position of an ear of the user, such as the location ofthe entrance of the auditory channel. The head tracking system 26 canalso be integrated in the headrest 24 so as to provide an integratedsystem. The position of the user's head 30 is detected or computed bythe position detection unit 46 of the head tracking system 26.

The head tracking is suitable for establishing the noise reduction area14 in that it is directly adjacent to the passenger's head 30, i.e. nearto the passenger's ears. When making reference to a noise reduction area14, it should be noted that there is a right noise reduction area 14 band a left noise reduction area 14 a, which are established so as toprovide a suitable noise reduction for both ears of the user. By way ofan example and without limitation, for the purpose of simplification ofexplanations only, reference will be made to a noise reduction area 14in the following. Notwithstanding the explanations are made for a singlenoise reduction area 14, the noise reduction system 20 is suitable forestablishing two or even more noise reduction areas 14 for at least bothears of a passenger or even for a plurality of passengers.

In an attempt to establish the noise reduction area 14 at the mostsuitable position for efficient noise reduction, the noise reductionsystem 20 applies the concept of virtual microphones 32. The virtualmicrophone 32 is established in the noise reduction area 14. At aposition of the virtual microphone 32, an error function is detected,which is the residual noise at the position of the virtual microphone 32after noise cancelation. By minimizing the error function at theposition of the virtual microphone 32, the noise reduction system 20optimizes noise-canceling performance. This is why it is desirable toplace the virtual microphone 32 as near to the entrance of the auditorychannel of the passenger's head 30 as possible. This can be performed byfor example relocating the position of the virtual microphone 32 basedon data generated by the head tracking system 26.

The control unit 10 runs a virtual sensing algorithm which is commonlyreferred to as the “remote microphone technique”. Without prejudice,reference will be made to this type of algorithm in the following.According to further embodiments, alternative algorithms can be run onthe control unit 10. These are for example algorithms referred to as:“virtual microphone arrangement”, “forward difference predictiontechnique”, “adaptive LMS virtual microphone technique”, “Kalmanfiltering virtual sensing” or “stochastically optimal tonal diffusefield virtual sensing technique”.

FIG. 3 is a drawing illustrating a noise reduction system 20 accordingto an embodiment. The system 20 comprises the sensor 8 detecting thebackground noise of the noise source 6. The background noise isconverted to a noise signal S, which is input to a dynamic adjustmentunit, which is configured to update parameters of an anti-noise unit 34,which is for generating of an anti-noise signal A. The anti-noise signalA is for driving the sound generator 12 in that it emits the anti-noisefor superposition with the background noise of the noise source 6 in thenoise reduction area 14. By way of an example only, this is illustratedin FIG. 3 and the following figures for only one ear of the passenger'shead 30. Furthermore, there is the dynamic adjustment unit 36 forupdating parameters of the anti-noise filter unit 34 based on an averageerror signal EA and so as to minimize the average error signal EA in anattempt to optimize the noise-canceling effect.

The noise reduction system 20 furthermore comprises the microphone array16, which comprises a plurality of monitor microphones 15 eachillustrated using a dot. The microphone array 16 is configured to pickup background noise and anti-noise for a plurality of virtual microphonepositions P1, P2 . . . PN. The virtual microphone positions are referredto as P1, P2 . . . PN for an arbitrary number of N of virtualmicrophones 15. The virtual microphone positions are generally alsoreferred to as P. They are located in the noise reduction area 14 andthey can be arranged in a grid, by way of an example only.

A maximum distance between the positions P actually depends on thefrequency range in which the noise-canceling algorithm operates. Thisfrequency range can be between 50 Hz and 600 Hz. The upper limit orcutoff frequency is chosen in that a prefix of the anti-noise signaldoes not invert within the noise reduction area 14. This prerequisite isfor the stability of the noise-canceling algorithm. When calculating aspatial distance from this frequency, this results in a maximum spatialdistance of about 0.2 m. This limit should be a maximum distance for thepoints P at which the virtual microphones are arranged. The same appliesfor a maximum distance between the point P at which the virtualmicrophone can be arranged, i.e. one of the aforementioned points P1 . .. PN and the physical position of the direct microphone 48, which willbe explained in detail further below.

The frequency range can be set by integrating a band pass unit 50 in thesignal line(s) of the either one or both of the noise signal S and theaverage error signal EA. The band pass unit 50 is illustrated in FIG. 3using a dashed line so as to illustrate that it is an optional unit. Itcan be implemented at a same position in all other embodiments.

In FIG. 3 , the control unit 10, which comprises the anti-noise unit 34and the dynamic adjustment unit 36, further comprises an averaging unit44, which is configured to calculate the average error signal EA. Theaverage error signal EA is indicative of a difference between thebackground noise and the anti-noise at more than one position P in thenoise reduction area 14. The dynamic adjustment unit 36 updates theparameters of the noise-canceling algorithm running in the anti-noiseunit 34 based on and so as to minimize the average error signal EA.

The estimation of the average error signal EA reflects more than oneposition P in the noise reduction area 14. It can be either performed bycalculating more than one error signal or by calculating an averageerror signal, which is indicative of a difference between the backgroundnoise and the anti-noise in a predetermined section PQ of the noisereduction area 14, wherein the section PQ comprises more than oneposition P. The first concept will be explained in the following bymaking reference to FIGS. 3 and 4 , the second concept will be explainedby making reference to FIGS. 5 and 6 . Naturally, multiple embodimentsof each respective concept are explained when making reference to thefigures.

Referring back to FIG. 3 , the control unit 10 further comprises a firstfilter unit 38, which is configured to receive the anti-noise signal A.The first filter unit 38 estimates a shifted anti-noise signal,generally referred to as A(x), which is indicative of the anti-noise atthe physical position x of one of the monitor microphones 15 of themicrophone array 16. By way of an example, the physical positions of themonitor microphones 15 are denoted x1 . . . x4. The correspondingshifted anti-noise signals for these positions x1 . . . x4 are A(x1),A(x2), A(x3) and A(x4). The shifted anti-noise signal A(x) representsthe estimated anti-noise signal at the respective physical position ofthe monitor microphones 15. For the calculation of the individualsignals A(x1), A(x2), A(x3) and A(x4), the first filter unit 38 cancomprise respective subunits.

Furthermore, the control unit 10 comprises a first arithmetic unit 39.The first arithmetic unit 39 receives the shifted anti-noise signalsA(x) and a monitor signal, generally referred to as N(X), of the monitormicrophones 15 being located at the physical position x. The firstarithmetic unit 39 can receive the shifted anti-noise signals A(x1),A(x2), A(x3) and A(x4) and the monitor signal N(x1 . . . x4) of themonitor microphones 15 being located at positions x1 . . . x4. The firstarithmetic unit 39 is configured to calculate a residual signal, whichis generally denoted R(x) and which is a difference between the monitorsignal N(x) and the shifted anti-noise signal A(x) at the physicalposition x of the monitor microphone 15. The first arithmetic unit 39can calculate the residual signals R(x1), R(x2), R(x3) and R(x4), whichis a respective difference between A(x1) and N(x1), A(x2) and N(x2),A(x3) and N(x3) and A(x4) and N(x4). The residual signal R(x) is theresidual noise at the respective position x of the monitor microphone15, which means the noise generated by the noise source 6 minus theanti-noise signal at a respective position x.

The residual signals R(x) are input to a second filter unit 40. Thesecond filter unit 40 is configured to estimate a shifted residualsignal R(P), which is the residual signal R(x) shifted to the position Pof the virtual microphone. Residual signals R(P1) . . . R(N) for arespective one of the position P1 . . . PN, such as for all thepositions P in the noise reduction area 14, can be calculated.

The control unit 10 further comprises a third filter unit 41, whichreceives the anti-noise signal A. The third filter unit 41 is configuredto estimate a shifted anti-noise signal, which is generally denoted A(P)and which is indicative of the anti-noise at the position P of thevirtual microphone 32. For calculation of a respective one of theshifted anti-noise signals A(P1) . . . A(PN), the third filter unit 41can comprise respective subunits.

Furthermore, the control unit 10 comprises a second arithmetic unit 42,which receives the residual signals R(P) and the shifted anti-noisesignals A(P), respectively. The second arithmetic unit 42 can receivethe shifted residual signals R(P1) . . . R(PN) and the shiftedanti-noise signals A(P1) . . . A(PN) for a respective one of thepositions P1 . . . PN in the noise reduction area 14. The secondarithmetic unit 42, from a respective one of these pairs of values,calculates or estimates an error signal, which should be generallydenoted E(P), for the position P of the virtual microphone. A firsterror signal E(P1) can be calculated for a point P1, a second errorsignal E(P2) is calculated for a point P2, wherein this is continued upto the maximum number N of points P in the noise reduction area 14,which means the error signal E(PN).

All the error signals E(P1) . . . E(PN) are input to the averaging unit44. From the error signals E(P), the averaging unit 44 calculates theaverage error signal EA. The average error signal EA can be thearithmetic average of all the previously mentioned error signals E(P1),E(P2) . . . E(PN). This averaging is performed at least for the firstand the second position P1, P2 of the virtual microphones. The averagingunit 44 can be configured to compute the average error signal EA, whichis the average of every error signals E(P1), E(P2) . . . E(PN) for allpositions P1, P2 . . . PN of the virtual microphones located in thenoise reduction area 14. The average error signal EA is input to thedynamic adjustment unit 36 to update parameters of the anti-noise filterunit 34, which means the updated parameters are calculated based oninformation about the average error signal EA and so as to minimize theaverage error signal EA. This leads to the effect of minimization ofbackground noise generated by the noise source 6 in the noise reductionarea 14.

The averaging unit 44 can be configured to calculate the average errorsignal EA from an arithmetic average of the individual error signalsE(P1), E(P2) . . . E(PN). According to another embodiment, the averagingunit 44 of the noise reduction system 20 is configured to calculate theaverage error signal EA as a weighted average. This can be performed bygiving one or more of the error signals E(P1), E(P2) . . . E(PN) anindividual weight or weighting factor. When calculating this weightedaverage, particular emphasis can be put on a certain point P, at which amain virtual microphone is located. For example, if the head 30 of thepassenger is in the position illustrated in FIG. 3 , the point PX islocated nearest to the ear of the passenger. Consequently, the bestperformance of the noise reduction should be at this particular pointPX. Hence, an overweight can be placed on the error function E(PX) forthe point PX and the corresponding virtual microphone. This can beperformed by for example giving the error function a higher weightingfactor than the remaining error functions of the other points P.

The location of the point PX, which is located nearest to the user'sear, can be performed by for example the head tracking system 26. Forthis purpose, the head tracking system 26 (see FIG. 2 ) comprises notonly the camera arrangement, comprising the stereo cameras 28, but alsothe position detection unit 46. The position detection unit 46 isconfigured for detecting a position and/or orientation of the head 30 ofthe user in the passenger transport area 4. The control unit 10 of thenoise reduction system 20 is than configured to select position PX as amain virtual microphone position, which is by way of an example only theposition referred to as PX. This selection can be made out of theplurality of predetermined positions P1, P2 . . . PN of the virtualmicrophones in the noise reduction area 14. However, it is also possibleto determine the position PX while disregarding the grid in which theremaining positions P1, P2 . . . PN are arranged. The main microphoneposition PX can be the position adjacent to an estimated position of anear of the user. The averaging unit 44 is configured to overweight theerror signal E(PX) of this main virtual microphone position PX whencalculating the average error signal EA.

There is a further embodiment of the noise reduction system 20, which isillustrated in FIG. 4 . This system 20 comprises a microphone array 16having a direct microphone 48. The parts and units of the system 20having the same reference numerals have already been explained whenmaking reference to FIG. 3 . The arrangement and functionality of theunits is similar. Unlike the system in FIG. 3 , the averaging unit 44 isconfigured to calculate the average error signal EA by further takinginto account a direct residual signal R(xd) of the direct microphone 48.

The first filter unit 38 can be configured to estimate a shifted directanti-noise signal A(xd). This signal A(xd) is indicative of theanti-noise at the physical position xd of the direct monitor microphone48. Furthermore, the first arithmetic unit 39 is configured to receivethe shifted direct anti-noise signal A(xd) and direct monitor signalN(xd) of the direct monitor microphone 48. The unit calculates a directresidual signal R(xd) from the difference of the direct monitor signalN(xd) and the shifted direct anti-noise signal A(xd), for the positionxd of the direct monitor microphone 48. The second filter unit 40 andthe second arithmetic unit 42 bypass the direct residual signal R(xd).The averaging unit 44 calculates the average error signal EA from theaverage of the error signals R(P1) . . . R(PN) for the positions P1 . .. PN in the noise reduction area 14 by further taking into account thedirect residual signal R(xd). By further taking into account the directresidual signal R(xd), the stability of the noise-canceling in the noisereduction area 14 is enhanced.

FIG. 5 shows a noise reduction system 20 according to a furtherembodiment. Units of this embodiment having the same reference numeralsas in FIGS. 3 and 4 are not repeatedly explained. The control unit 10comprises an averaging unit 44, which is unlikely the before explainedembodiments configured to receive a plurality of monitor signals N(X) ofthe monitor microphones 15 being located at different physical positionsx and to estimate an area monitor signal N(xq). This area monitor signalN(xq) is indicative of an error signal captured by the monitormicrophones 15 for a predetermined area xq of the monitor microphones15. The first filter unit 38 is configured to receive the anti-noisesignal A and to estimate a shifted area anti-noise signal A(xq). Thissignal is indicative of the anti-noise in the predetermined area xq. Thefirst arithmetic unit 39 receives the shifted area anti-noise signalA(xq) and the area monitor signal N(xq). The first arithmetic unit 39calculates an area residual signal R(xq), which is the differencebetween the area monitor signal N(xq) and the shifted area anti-noisesignal A(xq). The second filter unit 40 receives the area residualsignal R(xq) and estimates a shifted area residual signal R(PQ). Theshifted area residual signal R(PQ) is the area residual signal R(xq)shifted to a predetermined virtual area PQ, which comprises more thanone position P of the virtual microphones 32. The predetermined virtualarea PQ is exemplarily illustrated as a subarea or section of the noisereduction area 14.

The third filter unit 41 receives the anti-noise signal A and estimatesa shifted area anti-noise signal A(PQ), which is indicative of theanti-noise in the predetermined virtual area PQ. The averaging unit 44further comprises the second arithmetic unit 42, which is configured toreceive the shifted area residual signal R(PQ) and the shifted areaanti-noise signal A(PQ). The second arithmetic unit 42 further estimatesthe error signal E(PQ) for the predetermined virtual area PQ as theaverage error signal EA. The average error signal EA is again feedbackto the dynamic adjustment unit 36 so as to adapt or optimize theparameter of the anti-noise unit 34.

The concept of the area calculation of the monitor signal N, theresidual signal R and the anti-noise signal A can also be supplementedby further taking into account the signal of a direct microphone 48.This will be explained by making reference to the embodiment in FIG. 6 .

Units of the embodiment shown in FIG. 6 , which are given the samereference numerals as in FIG. 5 will not be explained repeatedly. Unlikethe embodiment in FIG. 5 , the averaging unit 44 receives the monitorsignals from the monitor microphones 15 being located at the positionsx1 . . . x3 and of the direct monitor microphone 48. One of the monitormicrophones 15 can be selected as the direct microphone 48, which shouldbe located at position xd. Hence, the direct monitor signal N comprisessignals of the monitor microphones 14 forming the monitor-microphonearray 16 and in addition to this the signal of the direct microphone 48being located at position xd, the signal is referred to as N(x1 . . .x3, xd).

The averaging unit 44 calculates from this signal the area monitorsignal N(xq), wherein the signals of the monitor microphones 15 beinglocated for example at positions x1 . . . x3 are taken into account.Furthermore, the averaging unit 44 forwards the direct monitor signalN(xd) to the first arithmetic unit 39. At the first arithmetic unit 39,as already explained with reference to the embodiment shown in FIG. 5 ,a difference between the area monitor signal N(xq) and the shifted areaanti-noise signal A(xq) is calculated and further processed as the arearesidual signal R(xq) by the second filter unit 40. The furtheroperation of the third filter unit 41 and the second differential unit42 is similar to the embodiment in FIG. 5 . Unlike this embodiment, thefirst arithmetic unit 39 is further configured to calculate a directresidual signal R(xd) from a difference of the direct monitor signalN(xd) and the shifted direct anti-noise signal A(xd). This directresidual signal R(xd) bypasses the second filter unit 40 and the seconddifferential unit 42 and is directly input into the averaging unit 44.The averaging unit 44 is configured to calculate the average errorsignal EA, which is an average of the error signal for the predeterminedvirtual area E(PQ) and the direct residual signal R(xd).

The various units described as part of the control unit 10 in FIGS. 3-6can be implemented as a single controller configured to perform each ofthe functions of the various units therein or as separate controllers orcomputing modules within the control unit 10 and can each be configuredas dedicated hardware circuits or software implemented on hardwarecontrollers/computing modules.

While there has been shown and described what is considered to beembodiments of the invention, it will, of course, be understood thatvarious modifications and changes in form or detail could readily bemade without departing from the spirit of the invention. It is thereforeintended that the invention be not limited to the exact forms describedand illustrated, but should be constructed to cover all modificationsthat may fall within the scope of the appended claims.

TABLE OF REFERENCE SIGNS

-   -   2 vehicle    -   4 passenger transport area    -   6 noise source    -   8 reference sensor    -   10 control unit    -   12 sound generator    -   14 noise reduction area    -   14 a left noise reduction area    -   14 b right noise reduction area    -   15 monitor microphone    -   16 monitor-microphone array    -   20 noise reduction system    -   22 seat    -   24 headrest    -   26 head tracking system    -   28 stereo cameras    -   30 head    -   32 virtual microphone    -   34 anti-noise unit    -   36 dynamic adjustment unit    -   38 first filter unit    -   39 first arithmetic unit    -   40 second filter unit    -   41 third filter unit    -   42 second arithmetic unit    -   44 averaging unit    -   46 position detection unit    -   48 direct monitor microphone    -   50 band pass unit    -   S noise signal    -   A anti-noise signal    -   N monitor signal    -   R residual signal    -   E error signal    -   P virtual microphone position    -   PQ predetermined virtual area    -   EA average error signal    -   PX main virtual microphone position    -   ED direct error signal    -   x physical microphone position    -   xq predetermined area    -   A(x) shifted anti-noise signal    -   A(xd) shifted direct anti-noise signal    -   A(xq) shifted area anti-noise signal    -   N(x) monitor signal    -   N(xd) direct monitor signal    -   N(xq) area monitor signal    -   R(x) residual signal    -   R(xd) direct residual signal    -   R(xq) area residual signal    -   R(P) shifted residual signal    -   R(PQ) shifted area residual signal    -   A(P) shifted anti-noise signal    -   A(Pq) shifted area anti-noise signal    -   E(P) error signal for point P    -   E(PQ) error signal for the virtual area PQ

What is claimed is:
 1. A noise reduction system for activelycompensating background noise generated by a noise source in a noisereduction area in a passenger transport area of a vehicle, the systemcomprising: a controller comprising hardware; a reference sensor fordetecting the background noise of the noise source; a sound generatorfor generating anti-noise for superimposing the anti-noise with thebackground noise in the noise reduction area for active reduction of thebackground noise; and a monitor-microphone array having a plurality ofmonitor microphones, the monitor-microphone array being disposedadjacent to the noise reduction area and being configured to pick upbackground noise emitted by the noise source and anti-noise emitted bythe sound generator; wherein a virtual sensing algorithm is implementedin the controller, the controller being configured to: estimate an errorsignal at a position of a virtual microphone, wherein the virtualmicrophone is located in the noise reduction area and the error signalis indicative of a difference between the background noise and theanti-noise at the position of the virtual microphone; generate ananti-noise signal for driving the sound generator in that it generatesthe anti-noise; calculate an average error signal, which is indicativeof a difference between the background noise and the anti-noise at morethan one position in the noise reduction area; and update parameters ofthe anti-noise unit based on the average error signal to minimize theaverage error signal.
 2. The noise reduction system according to claim1, wherein a plurality of positions are located in the noise reductionarea and the controller is configured to: estimate at least a firsterror signal for a virtual microphone located at a first position and asecond error signal for a virtual microphone located at a secondposition; and calculate the average error signal from at least the firstand the second error signals.
 3. The noise reduction system according toclaim 2, wherein the controller is further configured to calculate theaverage error signal, which is an arithmetic average of the at leastfirst and second error signals.
 4. The noise reduction system accordingto claim 2, wherein the controller is further configured to calculatethe average error signal, which is a weighted average of the at leastfirst and second error signals.
 5. The noise reduction system accordingto claim 4, wherein the controller is further configured to: detect aposition and/or orientation of a head of a passenger and to estimate aposition of an ear of a passenger in the passenger transport area;select a main position of the plurality of positions, which is adjacentto the estimated position of the ear of the passenger; and overweightthe error signal at the main position when calculating the average errorsignal.
 6. The noise reduction system according to claim 1, wherein thecontroller is further configured to: estimate a shifted anti-noisesignal, which is indicative of the anti-noise at a physical position ofone of the monitor microphones of the monitor microphone array;calculate a residual signal, which is a difference between a monitorsignal, of the monitor microphone being located at said physicalposition, and the shifted anti-noise signal at the physical position ofthe monitor microphone; estimate a shifted residual signal, which is theresidual signal shifted to the position of the virtual microphone;estimate a shifted anti-noise signal, which is indicative of theanti-noise at the position of the virtual microphone; and estimate theerror signal for the position of the virtual microphone by addition ofthe shifted residual signal and the shifted anti-noise signal.
 7. Thenoise reduction system according to claim 6, wherein a plurality ofpositions are located in the noise reduction area and the controller isconfigured to calculate and estimate the respective signals for at leastfirst and the second positions of the plurality of positions in thenoise reduction area.
 8. The noise reduction system according to claim7, wherein the controller is configured to calculate and estimate therespective signals for all of the plurality of positions in the noisereduction area.
 9. The noise reduction system according to claim 1,wherein the controller is configured to calculate an average errorsignal, which is indicative of a difference between the background noiseand the anti-noise in a predetermined area of the noise reduction areacomprising more than one position.
 10. The noise reduction systemaccording to claim 9, wherein the controller is configured to: receive aplurality of monitor signals of monitor microphones being located atdifferent physical positions and to estimate an area monitor signal,which is indicative of a monitor signal captured by the monitormicrophones for a predetermined area of the monitor microphones;estimate a shifted area anti-noise signal, which is indicative of theanti-noise in the predetermined area; calculate an area residual signal,which is a difference between the area monitor signal and the shiftedarea anti-noise signal; estimate a shifted area residual signal, whichis the area residual signal shifted to a predetermined virtual areacomprising more than one position of a virtual microphone; estimate ashifted area anti-noise signal, which is indicative of the anti-noise inthe predetermined virtual area; and estimate the error signal for thepredetermined virtual area as the average error signal, by addition ofthe shifted area residual signal and the shifted area anti-noise signal.11. The noise reduction system according to claim 1, wherein themonitor-microphone array further comprises a direct monitor microphoneand the controller is configured to calculate the average error signal,by further taking into account a direct residual signal of the directmonitor microphone.
 12. The noise reduction system according to claim 6,wherein: the monitor-microphone array further comprises a direct monitormicrophone and the controller is configured to calculate the averageerror signal, by further taking into account a direct residual signal ofthe direct monitor microphone; and the controller further comprising:estimate a shifted direct anti-noise signal, which is indicative of theanti-noise at a physical position of the direct monitor-microphone;calculate a direct residual signal, which is a difference between thedirect monitor signal, of the direct monitor microphone, and the shifteddirect anti-noise signal at the position of the direct monitormicrophone; and calculate the average error signal, which is an averageof the at least one error signal for a position in the noise reductionarea and the direct residual signal.
 13. The noise reduction systemaccording to claim 10, wherein: the monitor-microphone array furthercomprises a direct monitor microphone and the controller is configuredto calculate the average error signal, by further taking into account adirect residual signal of the direct monitor microphone; and thecontroller is further configured to: calculate a direct residual signal,which is a difference between the direct monitor signal, of the directmonitor microphone, and the shifted direct anti-noise signal at theposition of the direct monitor microphone; and calculate the averageerror signal, which is an average of the error signal for thepredetermined virtual area and the direct residual signal.
 14. The noisereduction system according to claim 1, wherein the controller is furtherconfigured to apply a band pass filter on the average error signaland/or on a noise signal picked up by the reference sensor for detectingthe background noise of the noise source.
 15. A method of operating anoise reduction system for actively compensating background noisegenerated by a noise source in a noise reduction area in a passengertransport area of a vehicle, the system comprising a controllercomprising hardware, a reference sensor for detecting the backgroundnoise of the noise source, a sound generator for generating anti-noisefor superimposing the anti-noise with the background noise in the noisereduction area for active reduction of the background noise, and amonitor-microphone array having a plurality of monitor microphones, themonitor-microphone array being disposed adjacent to the noise reductionarea and being configured to pick up background noise emitted by thenoise source and anti-noise emitted by the sound generator, wherein avirtual sensing algorithm is implemented in the controller, the methodcomprising: estimating an error signal at a position of a virtualmicrophone, wherein the virtual microphone is located in the noisereduction area and the error signal is indicative of a differencebetween the background noise and the anti-noise at the position of thevirtual microphone; generating an anti-noise signal for driving thesound generator in that it generates the anti-noise; calculating anaverage error signal, which is indicative of a difference between thebackground noise and the anti-noise at more than one position in thenoise reduction area; and updates parameters of the anti-noise unitbased on the average error signal and so as to minimize the averageerror signal.
 16. The method according to claim 15, wherein the methodfurther comprises: locating a plurality of positions in the noisereduction area; estimating at least a first error signal for a virtualmicrophone located at a first position and a second error signal for avirtual microphone (32) located at a second position; and calculatingthe average error signal from at least the first and the second errorsignals.
 17. The method according to claim 16, wherein the methodfurther comprises calculating the average error signal, which is anarithmetic average of the at least first and second error signal. 18.The method according to claim 16, wherein the method further comprisescalculating the average error signal, which is a weighted average of theat least first and second error signals.
 19. The method according toclaim 18, wherein the method further comprises: detecting a positionand/or orientation of a head of a passenger and estimating a position ofan ear of a passenger in the passenger transport area; selecting a mainposition of the plurality of positions, which is adjacent to theestimated position of the ear of the passenger; and giving an overweightto the error signal at the main position when calculating the averageerror signal.
 20. The method according to claim 15, wherein the methodfurther comprises: estimating a shifted anti-noise signal, which isindicative of the anti-noise at a physical position of one of themonitor microphones of the microphone array; calculating a residualsignal, which is a difference between a monitor signal of the monitormicrophone and the shifted anti-noise signal at the physical position ofthe monitor microphone; estimating a shifted residual signal, which isthe residual signal shifted to the position of the virtual microphone;estimating a shifted anti-noise signal, which is indicative of theanti-noise at the position of the virtual microphone; and estimating theerror signal for the position of the virtual microphone by adding theshifted residual signal and the shifted anti-noise signal.
 21. Themethod according to one of claim 20, wherein a plurality of positionsare located in the noise reduction area and the method further comprisescalculating and estimating the respective signals for at least the firstand the second positions in the noise reduction area.
 22. The methodaccording to claim 15, wherein method further comprises calculating anaverage error signal, which is indicative of a difference between thebackground noise and the anti-noise in a predetermined area of the noisereduction area comprising more than one position.
 23. The methodaccording to claim 22, wherein the method further comprises: receiving aplurality of monitor signals of monitor microphones being located atdifferent physical positions and estimating an area monitor signal,which is indicative of an error signal captured by the monitormicrophones for a predetermined area of the monitor microphones;estimating a shifted area anti-noise signal, which is indicative of theanti-noise in the predetermined area; calculating an area residualsignal, which is a difference between the area monitor signal and theshifted area anti-noise signal; estimating a shifted area residualsignal, which is the area residual signal shifted to a predeterminedvirtual area comprising more than one position of a virtual microphone;estimating a shifted area anti-noise signal, which is indicative of theanti-noise in the predetermined virtual area; and estimating the errorsignal for the predetermined virtual area as the average error signal byadding the shifted area residual signal and the shifted area anti-noisesignal.
 24. The method according to claim 15, wherein themonitor-microphone array further comprises a direct monitor microphoneand the method further comprises calculating the average error signal,by further taking into account a direct residual signal of the directmonitor microphone.
 25. The method according to claim 20, wherein themonitor-microphone array further comprises a direct monitor microphoneand the method further comprises calculating the average error signal,by further taking into account a direct residual signal of the directmonitor microphone; and the method further comprising: estimating ashifted direct anti-noise signal, which is indicative of the anti-noiseat a physical position of the direct monitor microphone; calculating adirect residual signal, which is a difference between a direct monitorsignal of the direct monitor microphone and the shifted directanti-noise signal at the position of the direct monitor microphone; andcalculating the average error signal, which is an average of the atleast one error signal for a position in the noise reduction area andthe direct residual signal.
 26. The method according to claim 23,wherein the monitor-microphone array further comprises a direct monitormicrophone and the method further comprises calculating the averageerror signal, by further taking into account a direct residual signal ofthe direct monitor microphone; and the method further comprises:calculating a direct residual signal, which is a difference between adirect monitor signal of the direct monitor microphone and the shifteddirect anti-noise signal at the position of the direct monitormicrophone; and calculating the average error signal, which is anaverage of the error signal for the predetermined virtual area and thedirect residual signal.
 27. The method according to claim 15, whereinthe method further comprises applying a band pass filter on the averageerror signal and/or on a noise signal picked up by the reference sensorfor detecting the background noise of the noise source.
 28. A processingapparatus for actively compensating background noise generated by anoise source in a noise reduction area in a passenger transport area ofa vehicle, the processing apparatus comprising: a controller comprisinghardware, the controller being configured to; implement a virtualsensing algorithm to estimate an error signal at a position of a virtualmicrophone, wherein the virtual microphone is located in the noisereduction area and the error signal is indicative of a differencebetween background noise of the noise source detected by a referencesensor and anti-noise generated by a sound generator for superimposingthe anti-noise with background noise in the noise reduction area foractive reduction of the background noise at the position of the virtualmicrophone; generating an anti-noise signal for driving the soundgenerator in that it generates the anti-noise; calculate an averageerror signal, which is indicative of a difference between the backgroundnoise and the anti-noise at more than one position in the noisereduction area; and update parameters of the anti-noise unit based onthe average error signal to minimize the average error signal.